Market Sizer

Market Sizer

The headline number is almost always the wrong number. A market with a huge Total Addressable Market and a tiny obtainable share is not a large opportunity, however good it looks on a slide. This skill sizes a market honestly, in three layers, using two independent methods so the estimate can be trusted.

The method

Size three layers:

• TAM (Total Addressable Market) — the maximum if you captured everything. This makes the

headline.

• SAM (Serviceable Addressable Market) — the portion you could realistically serve given your

model and geography.

• SOM (Serviceable Obtainable Market) — the portion you could realistically capture given

competition and your current position. This is what actually matters.

Estimate the market two ways and reconcile them, because a single method hides its own errors:

• Top-down — start from a large published figure and narrow by segment, geography, and

addressability. Search the web for a current, citable anchor rather than recalling one from memory; a sizing built on a stale or misremembered headline number inherits its error in every layer below.

• Bottom-up — start from units: number of potential customers × adoption × price × frequency.

Run the arithmetic programmatically (a quick script or calculator) rather than in your head; chained multiplications are exactly where sizings quietly go wrong.

If the two methods diverge by more than roughly 2x, that gap is information; find which assumption is doing the damage before you trust either number. State every major assumption and give a confidence level.

Output format

• Top-down estimate: the chain of figures and where each came from (with sources when

searched).

• Bottom-up estimate: the unit build-up.
• TAM / SAM / SOM: the three layers, with SOM as the headline conclusion.
• Reconciliation & confidence: why the two methods agree or differ, key assumptions, and how

confident to be.

How to run it

Default to producing the estimate: both methods and the three layers, never handing back a single TAM figure without its assumptions and a SOM. Switch to coaching when the user signals they want to build it: ask them for the inputs to each method and help them reconcile; the reconciliation is the skill.

Where this breaks

Market sizing is only as good as its assumptions, and for genuinely new markets there is no reliable base rate, so the estimate is a structured guess, not a measurement; presenting a precise figure for an unknowable market is false confidence. Top-down anchoring on a big published TAM also biases the whole estimate upward. Show ranges, not false precision, when the inputs are soft.

Style

Plain language, define TAM/SAM/SOM on first use, no em dashes, short paragraphs. Lead the conclusion with SOM, not TAM.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: AnugamChakra
• Source: AnugamChakra/think-like-a-strategy-consultant
• License: MIT

Install and usage instructions live in the source repository linked above.